The recorded image of an object imaged by conventional imaging system is typically sharply in focus only over a limited object distance range (in front of and behind the object) known as the ‘depth of field’ (DOF). The DOF is inversely proportional to the square of the numerical aperture of the imaging system for diffraction-limited imaging. On the flip-side, the ‘depth of focus’ is the amount by which the image may be shifted in a longitudinal direction and introduce no more than acceptable blur. Present-day cameras have mechanical focusing means (including automatic systems), to provide high quality images of particular object scenes at various object distances. Even with these focusing means it can be difficult to sharply photograph object scenes that span large axial distances (i.e., those exceeding the DOF of the optical system). Cameras with a larger depth of focus will provide better imaging performance over those without with respect to depth of field.
Digital processing of image data on a pixel-by-pixel basis has afforded more opportunity for improving and correcting optically imaged scenes. Some of these improvements have related to increasing the depth of field in the recorded image. For example, digital processing has been used to combine images of the same scene taken at different depths of focus to produce a composite image that has recorded an extended depth of field (or, as may be said, having an extended depth of field). The multiple images take time to collect, are difficult to process, and are generally unsatisfactory for scenes subject to change.
Amplitude attenuation filters have also been used to extend the depth of field. Typically, the attenuation filters are located in the aperture of the imaging system, leaving inner radii clear but attenuating the outer annulus. Moreover, these filters tend to introduce large amounts of light loss, which limits their applications.
More promising attempts have been made that deliberately blur an intermediate image in a systematic way so that at least some information about the imaged object is retained through a range of focus positions and a non-ideal impulse response function remains substantially invariant over the defocus range. Digital processing, which effectively deconvolutes the point spread function, restores the image to a more recognizable likeness of the object through an extended depth of field.
One such example locates a cubic phase mask within the aperture of the imaging system to generate a distance invariant transfer function. Digital processing removes the blur. Although significant improvement in the recorded depth of field is achieved, the cubic phase mask is not rotationally symmetric and has proven to be expensive and difficult to fabricate.
Another such example similarly locates a circularly symmetric, logarithmic asphere lens to extend the depth of field. Although this solution is more economical to manufacture, the impulse response has not been found to be substantially uniform over the full range of operation and, as a result, some degradation is experienced in the image quality of the recovered image.
Reconstruction algorithms for removing the blur of such intermediate images are subject to problems relating to the quality and efficiency of their results. Nonlinear processing algorithms can suffer from slow convergence or stagnation and produce images with reduced contrast at high spatial frequencies.
In view of the aforementioned challenges and shortcomings associated with conventional extended depth of field imaging apparatus and processes, and others appreciated by those skilled in the art, the inventors have recognized the unfilled need for apparatus and methods that are better capable of, and cost effective at, providing better images that exhibit extended depths of field.